This invention relates to a belt for a shoe press in the press part of a papermaking machine. In particular, the invention relates to improvements in the durability of a shoe press belt.
Shoe presses used in recent years in the press parts of papermaking machinery are classified roughly into the two types shown in FIGS. 8 and 9. Each of these shoe presses includes a press roll P and a shoe S. The shoe S has an arcuate top recess conforming with the bottom of a roll P. An endless upper felt 92 and an endless lower felt 93 run between the roll P and the shoe S, and a wet web W runs between the felts 92 and 93. An endless press belt B runs between the lower felt 93 and the shoe S, and follows the felt 93. The shoe S urges the belt B upward to press the felts 92 and 93 against the roll P. The effect of the shoe is to form a wide nip zone, which improves the water squeezing effect of the pressure exerted between the roll P and the shoe S.
The press belt B of the shoe press 91 shown in FIG. 8 is relatively long, and runs under constant tension over guide rollers 94, there being four such guide rollers in the apparatus of FIG. 8. The press belt B of the shoe press 91 shown in FIG. 9 is relatively short, and its path is a small circle.
In general, as shown in FIG. 10, the press roll P of each shoe press 91 includes both a cylindrical central portion, and end portions Pxe2x80x2 tapering in the axial outward direction. The shoe S of each shoe press 91 has chamfered round ends Sxe2x80x2.
In the older shoe presses of the foregoing types, the nip zone N, that is, the zone underneath the middle portion of the press roll not including the tapered end portions Pxe2x80x2, was wider than the shoe width Sxe2x80x3, the shoe width being the width of the middle portion of the shoe, not including the chamfered round ends Sxe2x80x2. On the other hand, in some current shoe presses, as shown in FIG. 10, the nip zone N formed between the roll P and the shoe S is narrower than the shoe width Sxe2x80x3. The zones above the end portions of the shoe outside the nip zone N may be referred to as non-nip zones Nxe2x80x2.
In the shoe press shown in FIG. 10, a shearing force acts continuously on the press belt B at the boundaries Pxe2x80x3, between the nip zone N and the non-nip zones Nxe2x80x2, at the inner ends of the tapered end portions Pxe2x80x2 of the press roll. As a result of the shearing force, cracks are liable to develop along the boundary Pxe2x80x3 in the belt B. These cracks adversely affect the durability of the press belt, making it unsuitable for long-term use.
In view of the foregoing problem, the object of this invention is to provide a durable shoe press belt, in which the shearing force acting at the boundary between the nip zone and the non-nip zones is either non-existent or sufficiently weak that it causes minimal damage to the press belt.
In accordance with the invention, a specially constructed shoe press belt is used in a shoe press which comprises a press roll and a shoe, and in which the nip zone formed between the press roll and the shoe is narrower than the shoe. In this press, therefore, there is a nip zone, and at least one non-nip zone, meeting at a boundary, which will ordinarily coincide with the outer end of a cylindrical part of the press roll. The press belt, which extends through the nip zone and the non-nip zone, has a body comprising a hard portion and a portion of less hardness, and a part of the portion of the belt having less hardness is directly opposite the boundary between the nip and non-nip zones.
In one preferred embodiment of the invention, the hard portion of the belt body comprises a high molecular weight elastic material, and the portion of the belt body having less hardness comprises a high molecular weight elastic material having a hardness less than that of the high molecular weight elastic material of the hard portion of the body.
In another preferred embodiment, the hard portion of the body comprises a dense, high molecular weight, elastic material, and the portion of the body having less hardness comprises a high molecular weight elastic material having voids formed therein.
In still another embodiment, the belt body comprises a thin portion and a thick portion, so that a part of the thin portion can be positioned directly opposite the boundary between the nip and non-nip zones.